In the development of internal combustion engines, it is constantly sought to minimize the fuel consumption and to reduce pollutant emissions, in particular the emissions of nitrogen oxide.
Fuel consumption is problematic especially in the case of spark ignition engines. The reason is the fundamental principle of operation of the conventional spark ignition engine, which is operated with a homogeneous fuel/air mixture, with the desired power output being adjusted by changing the filling of the combustion chamber, i.e. by means of quantity control. By adjusting a throttle valve provided in the intake tract, the pressure of the induced air downstream of the throttle valve can be reduced to a greater or lesser extent. Due to the throttling losses, the quantity control has thermodynamic disadvantages, especially in the part load range.
Injecting fuel directly into the combustion chamber of the at least one cylinder is regarded as a suitable measure for noticeably reducing fuel consumption, even in spark ignition engines. Dethrottling of the internal combustion engine is accomplished by using quantity controls in certain limits, as with the diesel engine.
By direct injection of the fuel into the combustion chamber, it is possible, in particular, to achieve a stratified charge in the combustion chamber, and this can make a significant contribution to the dethrottling of the spark ignition operating method since the mixture fed to the internal combustion engine can be made very significantly leaner with the aid of operation with a stratified charge.
For the reduction of the nitrogen oxide emissions of an internal combustion engine, a distinction can be made between two fundamentally different approaches. In a first approach, it is sought to influence the combustion process such that the fewest possible nitrogen oxides arise, that is to say are formed, during the combustion of the fuel in the first place. Since the formation of the nitrogen oxides requires not only an excess of air, but also high temperatures, combustion processes with relatively low combustion temperatures, so-called low temperature combustion methods (LTC), are inter alia expedient for the reduction of the nitrogen oxide emissions.
Low combustion temperatures may be realized by virtue of the ignition retardation being increased, and the rate of combustion being reduced. Both can be achieved through the admixing of combustion gases to the cylinder fresh charge or by increasing the exhaust-gas fraction in the cylinder fresh charge, whereby exhaust-gas recirculation (EGR) is regarded as a suitable measure for lowering the combustion temperature, specifically both external exhaust-gas recirculation, that is to say the recirculation of combustion gases from the exhaust-gas side to the intake side of the internal combustion engine, and internal exhaust-gas recirculation, that is to say the retention of exhaust gases in the cylinder during the charge exchange. With increasing exhaust-gas recirculation rate, the nitrogen oxide emissions can be considerably reduced.
To obtain a significant or adequate reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates are required which may be of the order of magnitude of xEGR≈60% to 70%. Therefore, the hot exhaust gas is cooled during the course of the recirculation. The cooling of the recirculated exhaust gas facilitates, or permits, the realization of high recirculation rates. The lowering of the temperature of the exhaust gas during the course of the cooling leads to an increase in density, and to a smaller exhaust-gas volume for a given exhaust-gas mass. Furthermore, the cooling of the recirculated exhaust gas assists the lowering of the combustion temperature, because this also results in the temperature of the entire cylinder fresh charge being lowered. There is nevertheless a conflict in the case of internal combustion engines having exhaust-gas turbocharging and simultaneous use of exhaust-gas recirculation since the recirculated exhaust gas is regularly extracted from the exhaust-gas removal system upstream of the turbine and is then no longer available for driving the turbine of the exhaust-gas turbocharger, and therefore the torque characteristics suffer.
As a result of the measures described above, not only the nitrogen oxide emissions, but also the soot emissions are reduced.
Another concept for realizing combustion processes with relatively low combustion temperatures in order to reduce the nitrogen oxide emissions relates to water injection, in which, in addition to the fuel, water is additionally introduced into the cylinder. The water introduced into the fired cylinder is—like the fuel—heated and evaporated, and therefore the temperature of the gas mixture in the cylinder drops, in particular because of evaporation enthalpy. In addition, the volume of the water is considerably increased by means of the evaporation, as a result of which the pressure in the cylinder rises. The latter increases the work output by the cylinder to the crankshaft or increases the power.
In contrast thereto, as a result of the course of combustion, conventional low temperature processes have on the contrary thermodynamic disadvantages, and therefore the efficiency is lower than in the case of a combustion process which is optimized exclusively in respect of the fuel consumption, i.e. in respect of the efficiency, without consideration of the pollutants emitted.
DE 199 55 344 B4 describes an assembly for injecting fuel into the cylinder of a direct-injection internal combustion engine, with which water can be additionally introduced into the cylinder. A membrane forms a nonreturn valve which permits water to flow into the cylinder via spray holes when a certain system pressure is exceeded, but does not permit the water to flow back. However, the pretensioned membrane does not permit a reproducible injection operation of water with a defined opening duration, which is determined by an opening time and a closing time. In addition, the durability of the membrane is a problem.
However, direct-injection internal combustion engines are in principle highly sensitive to changes and deviations in the mixture formation, in particular with regard to the quantity of fuel or quantity of water injected. There is comparatively little time for the injection of the fuel and water, the mixture preparation in the internal combustion engine, namely the preparation of the fuel and water by evaporation, and the thorough mixing of air, fuel vapor and water vapor, and also the ignition of the prepared mixture. For this reason too, very precise metering, i.e. proportioning of the quantity of water introduced, is absolutely essential. Unwanted deviations in the quantity of water injected can disadvantageously influence the operation of the internal combustion engine and even lead to increased pollutant emissions. In addition, rotational speed fluctuations of the internal combustion engine, misfires and backfires may occur.
A second approach for reducing the nitrogen oxide emissions consists in aftertreatment of the exhaust gas that is formed during the combustion, and of the pollutants contained therein. According to the prior art, to reduce the pollutant emissions, internal combustion engines may be equipped with various exhaust-gas aftertreatment systems. The exhaust-gas aftertreatment is also associated with disadvantages.
Firstly, the aftertreatment of exhaust gases is expensive, particularly due to the required coating of the catalytic converters with high-grade metals, wherein different aftertreatment systems regularly have to be provided for different pollutants, which may also lead to packaging problems.
Secondly, the operation of the exhaust-gas aftertreatment systems as such is associated with disadvantages, for example the use of fuel for maintaining the functionality of the exhaust-gas aftertreatment systems.
In one embodiment, the internal combustion engine is a supercharged internal combustion engine which is distinguished by high cylinder pressures and high exhaust-gas temperatures.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and in a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower. Supercharging consequently assists in the continuous efforts in the development of internal combustion engines to minimize fuel consumption, i.e. to improve the efficiency.
As one example, the issues described above may be addressed by a direct-injection supercharged internal combustion engine having water injection and comprising, at least one cylinder head comprising at least one cylinder, in which each cylinder is assigned an injection nozzle which: is at least connectable to a fuel reservoir which serves for storing fuel; is secured in a nozzle holder serving as a receptacle; and is fitted with a nozzle needle which is displaceable in the direction of a longitudinal axis in a nozzle needle guide and, opens up at least one nozzle hole in order to introduce fuel, wherein a control piston is provided, which is mounted movably on the injection nozzle; is displaceable in a translatory manner along the longitudinal axis of the injection nozzle between an inoperative position and a working position; and closes at least one fluid connection in the inoperative position and opens up the at least one fluid connection in the working position in order to introduce water into the associated cylinder, wherein the control piston connects the at least one fluid connection in the working position to a chamber which, as part of a water supply system, is at least connectable via a supply line to a water reservoir which serves for storing water.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.